Ranging at high accuracy is a core problem in many technical fields. For example, high precision ranging can increase efficiency and productivity in agriculture by using robots for pruning, weeding and crop-spraying. In marine navigation, ranging with high accuracy can help to enter a port with big ships. Also the landing phase of an airplane can be automated by reliable high precision ranging techniques.
Ranging with radio systems is usually performed by measuring the propagation delay of an electro-magnetic wave with known structure. If the radio signal is transmitted on a high carrier frequency, it is understood that the carrier phase conveys significant information about the delay parameter. However, as the mapping between phase and delay parameter is ambiguous, it is believed that the carrier phase can only be exploited by combining measurements attained with different signal sources.
In the application of satellite-based synchronization and navigation (GPS, GLONASS, Galileo, etc.), high precision is therefore achieved by performing three independent steps, as illustrated in FIG. 1. An acquisition algorithm delivers some initial knowledge about the range between transmitter 1 and the receiver for all available transmitters l=1, . . . , L. Here this initial knowledge is characterized by a Gaussian random variable with mean μinit(l) and variance σinit2. For each transmitter an individual tracking module then measures and tracks the baseband delay and the carrier phase of the radio signal as independent parameters. In practice, this is done with two control loops, the delay-locked loop (DLL) and the phase-locked loop (PLL). With the DLL only a coarse ranging solution can be obtained. The carrier phase ζ(l) can be measured with much higher precision. However, the carrier phase is periodic with 2π and the measurement ζ(l), is, thus, only given by some fraction of a cycle, i.e. the integer number Ψ(l) of whole cycles is not known at the receiver
                    ϛ        ^                    (        l        )              =                            ω          c                ⁢                  τ                      (            l            )                              +              2        ⁢                  πΨ                      (            l            )                              +              e        ς                  (          l          )                      ,                    ϛ        ^                    (        l        )              =                            ω          c                ⁢                  τ                      (            l            )                              +              2        ⁢                  πΨ                      (            l            )                              +              e        ς                  (          l          )                      ,where
      w    c    =      2    ⁢    π    ⁢                  ⁢          f      c      is the carrier frequency,
      τ          (      l      )        =            r              (        l        )              c  is the propagation-delay, c velocity of light and eζ(l) the measurement error. Resolving the integer Ψ(l) precisely with measurements from one transmitter is not possible. Measurements from multiple transmitters and multiple time instances must be combined in order to resolve the ambiguity problem and obtain a ranging solution with high accuracy.
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The present invention aims to solve the above mentioned problems. In particular, the present invention aims to provide a method and apparatus that provides robust ranging at high accuracy and high processing rate.